The duration of action and cardiopulmonary effects of propofol (6.55 mg/kg intravenously), xylazine (0.8 mg/kg intramuscularly), medetomidine (30 micrograms/kg intramuscularly), xylazine plus propofol (3 mg/kg intravenously) and medetomidine plus propofol (3 mg/kg intravenously) were compared in dogs. A cannula inserted into a raised carotid artery before the drugs were given allowed the continuous recording of blood pressure and heart rate and the measurement of arterial pH, PCO2, PO2, bicarbonate and base balance. Xylazine and medetomidine premedication prolonged propofol anaesthesia in dogs. Propofol alone reduced blood pressure and transiently raised heart rate. The apnoea and hypoxaemia induced by propofol alone also occurred in the premedicated groups with hypoxaemia being most evident in the medetomidine/propofol group. Bradycardia was a common feature in all the dogs given xylazine or medetomidine, but hypertension was consistently recorded in all the dogs given medetomidine.